fix: handle non-string values in pytest print

[iamdefinitelyahuman]

What I did

When running pytest, understand non-string values in print.

Fixes an issue reported on Gitter.

[codecov-io]

Codecov Report

Merging #479 into master will decrease coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
- Coverage   88.26%   88.07%   -0.19%     
==========================================
  Files          56       56              
  Lines        6075     6081       +6     
  Branches     1335     1338       +3     
==========================================
- Hits         5362     5356       -6     
- Misses        493      502       +9     
- Partials      220      223       +3     

Impacted Files	Coverage Δ
brownie/test/managers/runner.py	86.08% <100.00%> (ø)
brownie/utils/color.py	82.10% <0.00%> (-3.16%)	⬇️
brownie/network/rpc.py	85.47% <0.00%> (-2.91%)	⬇️
brownie/network/web3.py	95.23% <0.00%> (-1.91%)	⬇️
brownie/project/main.py	90.76% <0.00%> (+0.12%)	⬆️

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